Synthesis gas contains carbon monoxide and hydrogen that can be further purified to produce hydrogen and carbon monoxide products or can be further reacted in such downstream chemical processes that, for example, involve the production of methanol or known gas to liquid processes for synthetic fuels by means of the Fischer-Thropsch process.
Synthesis gas is generated within a steam methane reformer by introducing a hydrocarbon containing feed, typically natural gas, into steam methane reformer tubes located in a radiant section of the steam methane reformer. The reformer tubes are packed with a catalyst that is used to promote the steam methane reforming reactions. Steam methane forming reactions are endothermic and hence, heat is supplied to the reformer tubes to support the reactions by burners firing into the radiant section of the steam methane reformer. Synthesis gas can also be generated in a partial oxidation reactor by reaction between hydrocarbon and oxidant (e.g. oxygen) or in an autothermal reformer by reaction between hydrocarbon, oxidant and steam.
After a synthesis gas stream has been cooled, the steam and carbon monoxide content of the synthesis gas can be further reacted in a water-gas shift reactor to increase the hydrogen content of the synthesis gas.
An integrated steam generation system is located within the synthesis gas plant to produce the steam for the steam methane reforming reaction, for the water-gas shift reaction and also, for export. The exported steam can itself constitute a valuable product that can affect the economic viability of the hydrogen plant. Steam methane reformers typically have convective heat exchange sections that are connected to the radiant sections. The heated flue gases produced by the burners firing into the radiant section are passed through the convection section to raise steam and to superheat steam for the purposes outlined above. The steam generation system also utilizes heat exchangers both upstream and downstream of the water-gas shift reactor. In this regard, the synthesis gas stream generated in the steam methane reformer must be reduced in temperature to a level suitable for the water-gas shift reactor and consequently, a heat exchanger located upstream of the water-gas shift reactor both cools the synthesis gas stream and raises some of the steam. Since the water-gas shift reaction is an exothermic process, the heat contained in the shifted stream is commonly utilized in heat exchangers located downstream of the water-gas shift reactor for the production of additional steam. All of such steam is routed to a steam header and then superheated in the convective section of the steam methane reformer.
The synthesis gas produced by the steam methane reforming reactions has a carbon dioxide content. After the water-gas shift reactor, the carbon dioxide content of the synthesis gas is further increased as a result of the reaction of the steam with the carbon monoxide. Separation of the carbon dioxide from the synthesis gas is often necessary for downstream processing of the synthesis gas, for example, in methanol production. Additionally, carbon dioxide itself is a valuable product. For example, enhanced oil recovery processes utilize carbon dioxide that is injected down hole in an injection well to drive oil to producing wells. In an enhanced oil recovery process, injection of carbon dioxide in the oil reservoir lowers the viscosity of oil, which allows oil to flow more easily and oil recovery from the reservoir is increased. It is to be noted, however, that when carbon dioxide is used for such purposes it has to be very pure and consequently, is not readily obtained from steam methane reforming plants. Additionally, carbon dioxide emissions from the use of hydrocarbons has been linked to global warming. To address this problem, it has been proposed that carbon dioxide be captured from the industrial sources and injected underground in brine aquifers or in deep oceans for permanent capture of carbon dioxide and thus stabilize atmospheric carbon dioxide levels.
Various carbon dioxide removal systems have, however, been integrated with steam methane reforming facilities in order to at the very least separate the carbon dioxide from the synthesis gas but also, to recover the carbon dioxide for sequestration purposes or for use as a value added product.
U.S. Pat. No. 5,000,925 describes a process to recover hydrogen and carbon dioxide from a hydrogen plant employing a steam methane reformer. In the process, a synthesis gas stream produced from the water-gas shift reactor is introduced into a hydrogen pressure swing adsorption unit to generate a product hydrogen stream and a tail gas stream. The tail gas is compressed and then separated using a carbon dioxide pressure swing adsorption unit to produce a hydrogen-rich stream and a carbon dioxide-rich stream. The carbon dioxide-rich stream is compressed and further purified in a cryogenic unit to produce liquid carbon dioxide. The hydrogen-rich stream is recycled back to the steam methane reformer.
U.S. Pat. No. 6,551,380 discloses a process in which a synthesis gas stream produced by a water-gas shift reactor within a hydrogen plant is conventionally introduced into a hydrogen pressure swing adsorption unit to recover hydrogen and thereby to produce a tail gas stream. The tail gas stream is compressed and then processed in an adsorption unit to recover the carbon dioxide. The carbon dioxide from adsorption unit is sent to the liquefier to produce a purified liquid carbon dioxide product and off-gas from the adsorption unit is sent to a second hydrogen pressure swing adsorption unit to produce hydrogen.
A problem in practically utilizing either of the processes set forth in the patents listed above arises from the common use of the tail gas stream as part of the fuel to the steam methane reformer. Any interruption in the fuel will cause the steam methane reformer system to go off-line resulting in a costly restart in which the primary fuel to the steam methane reformer, typically natural gas, must be utilized to bring the reformer back up to its operational temperature. Another problem in utilizing tail gas is that it must be compressed before a crude carbon dioxide stream can be separated in a vacuum pressure swing adsorption process. The compression step results in additional energy and capital costs.
As will be discussed, the present invention does not extract the carbon dioxide from the tail gas to thereby avoid the problem discussed above. Moreover, the present invention by virtue of the location of recovery of the carbon dioxide within the hydrogen plant has further advantages over the prior art.